1. Field of the Invention
The present invention relates to a heterojunction field effect transistor, which utilizes a two-dimensional electron gas layer formed at a heterojunction interface.
2. Description of the Prior Art
Conventionally, a HEMT (High Electron Mobility Transistor), shown in FIG. 1 is known. In the HEMT shown in FIG. 1, an undoped GaAs layer 2 and an n-type Al.sub.x Ga.sub.l-x As layer 3 are sequentially epitaxially grown on a semi-insulating GaAs substrate 1. A Schottky gate electrode 4, a source electrode 5 and a drain electrode 6 are formed on the n-type Al.sub.x Ga.sub.l-x As layer 3. The source electrode 5 and the drain electrode 6 reach the GaAs layer 2 through the n-type Al.sub.x Ga.sub.l-x As layer 3.
In the HEMT shown in FIG. 1, a heterojunction 7 is formed between the n-type Al.sub.x Ga.sub.l-x As layer 3 and the GaAs layer 2. A two-dimensional electron gas layer 8 is formed in a portion of the GaAS layer 2 adjacent to the heterojunction 7. This may be understood upon reference to an energy band diagram shown in FIG. 2. As there shown, an approximately inverted-triangular potential well is formed at a conduction band edge E.sub.c in the portion of the GaAs layer 2 adjacent to the heterojunction 7. Electrons migrating from the n-type Al.sub.x Ga.sub.l-x As layer 3 to the GaAs layer 2 due to an energy difference at the conduction band edge E.sub.c between the n-type Al.sub.x Ga.sub.l-x As layer 3 and the GaAs layer 2 are accumulated in this well, thus forming the two-dimensional electron gas layer 8. A step .DELTA. E.sub.c of the conduction band edge E.sub.c at an interface of the heterojunction 7 is about 0.3 eV when an Al composition ratio x is 0.3.
The HEMT shown in FIG. 1 is termed a forward HEMT. In addition to this type, a so-called reverse HEMT (whose energy band diagram is shown in FIG. 3), is known, wherein a stacking order of a GaAs layer and an n-type Al.sub.x Ga.sub.l-x As layer 3 is reversed. Furthermore, in order to increase a concentration n.sub.s of the two-dimensional electron gas layer 8, n-type Al.sub.x Ga.sub.l- As layers have been formed at both sides of a GaAs layer so as to obtain a so-called double heterojunction FET (whose energy band diagram is shown in FIG. 4) which combines forward and reverse HEMTs having a common GaAs layer.
The above-mentioned forward HEMT reverse HEMT, and double heterojunction FET have common structural characteristics wherein current paths run parallel to the substrate surface, and a high electron mobility at the two-dimensional electron gas layer formed in a portion of a GaAS layer adjacent to the heterojunction is utilized. Accordingly, the above-mentionedd forward HEMT, reverse HEMT, and double heterojunction FET still have a problem of low planar patterning precision as in a conventional GaAs FET. Due to this problem, their gate lengths cannot be shortened beyond a certain limit. In addition to these, the conventional HEMTs have a limited current density, thus limiting their performance.